What Is Blood-Type Personality?

You have probably been asked, on first meeting, "What is your blood type?" In East Asia, including Korea and Japan, blood type has long been loved as a warm conversation topic for lightly gauging someone's personality, beyond mere medical information. This current that talks about personality by sorting it into four — type A, type B, type O, and type AB — is called blood-type personality, and it has settled in like a joyful game for understanding friends and introducing oneself. When Gallup Korea polled 1,501 adults aged 19 and over in October 2023, roughly 57 percent said they believed blood type and personality are related. When nearly six people in ten nod along to a story, it is worth knowing where that story came from and where it stops.

An Important Point First

Let me make the most important point clear at the outset. Blood-type personality is not a scientifically proven theory. In 2014, a team led by Kengo Nawata reanalyzed large-scale, randomly sampled survey data covering more than 10,000 people across Japan and the United States. Of 68 attitude and personality items, 65 showed no significant difference between blood groups, and the three that did differ were so slight as to be trivial. The researchers concluded that the evidence strongly indicates no connection between blood type and personality; the paper appeared in the journal of the Japanese Psychological Association.

A second study is even more revealing. Akira Sakamoto and Kenji Yamazaki examined 32,347 responses from annual Japanese opinion polls run between 1978 and 1988. The correlation between blood-type stereotypes and respondents' self-reported personalities was not there from the beginning. It emerged only after 1984, the years when blood-type books and television segments exploded in popularity. In other words, blood type did not shape personality; people who had heard for years that "type A is meticulous" began describing themselves that way. Researchers call this contamination by knowledge, and it looks a great deal like a self-fulfilling prophecy.

One more piece belongs here. In 1948, the psychologist Bertram Forer handed his students what he described as individualized personality analyses. Every student received the identical text. Asked how well it fit them, they rated it 4.26 out of 5 on average. That tendency to accept a vague description that would fit anyone as one's own private story is what we now call the Barnum effect. When a blood-type description makes you think "how did they know?", that is the first name to recall.

From Transfusion Medicine to Temperament: A Century-Long Path

The ABO system itself is real science. In 1901 the Austrian physician Karl Landsteiner observed that red cells from different people clumped in different patterns, identifying the blood groups, and he received the Nobel Prize in Physiology or Medicine in 1930 for it. That is the discovery that made transfusion safe. The trouble is what happened next. In 1919, Ludwik and Hanka Hirszfeld published in The Lancet a survey of the blood of some 8,000 soldiers of many nationalities gathered at Salonika during the First World War, and proposed a so-called biochemical race index calculated as (A+AB)/(B+AB). British troops scored 4.5; Indian troops 0.5. Those numbers were promptly seized by people who wanted to rank human groups, and through the 1920s and 1930s European eugenic literature repeatedly tied type B to inferiority and criminality.

In Japan, the physician Kimata Hara raised a link between blood group and temperament in 1916, and popularization began in earnest in 1927 when Takeji Furukawa, a professor at Tokyo Women's Normal School, published "The Study of Temperament Through Blood Type" in the journal Psychological Research. The familiar template hardened right there: A introverted, B extroverted, O strong-willed, AB double-sided. The evidence behind it was an observation of eleven relatives, later widened to a survey of a few hundred acquaintances. After the Musha Incident of 1930 in colonial Taiwan, Furukawa examined the blood types of Taiwanese Indigenous people and, finding 41.2 percent type O, argued their rebelliousness was innate — contrasting them with the Ainu, whom he characterized as submissive and among whom type O ran to 23.8 percent. It is a plain case of a theory serving the needs of colonial administration. Japan's medical establishment formally dismissed the claim as without scientific basis in 1933.

The boom returned in 1971, when Masahiko Nomi, a writer with a background in broadcasting rather than medicine or psychology, published a book on compatibility by blood type that sold in the millions; his son later took over and built an industry around it. The idea reached Korea through comics and variety shows in the 1990s, and hit its popular peak with the film My Boyfriend Is Type B, released in February 2005 and seen by about 1.48 million viewers.

The Four Types, and Population Share as a Hidden Variable

The Korean figures come from a large genomic study consistent with Korean Red Cross blood service statistics; the Japanese figures are from nationwide samples. What deserves attention is the relationship between image and population share. In Japan, type A is the largest group and type B the smallest — and, as it happens, type A carries the virtues of the dependable majority while type B carries the mark of the oddball. The ordinary social grammar in which the majority becomes the standard and the minority becomes peculiar simply put on blood-type clothing. Korea's type B share exceeds 27 percent, distinctly higher than Japan's, and yet the "type B guy" joke was imported wholesale. That alone shows these images came out of stories, not data.

Blood Type Was Never Really Four Boxes

The edge cases are worth knowing. ABO does not divide cleanly into four. The most common ABO subgroup in Korea is cis-AB, a variant in which the ability to make both A and B antigens is inherited from one parent on a single gene. The first Korean case was reported in 1985, and the variant is far more frequent in East Asia than elsewhere. Its typical phenotype, A2B3, expresses the B antigen weakly, so depending on the testing conditions the same person may be read as type A or as type AB. In transfusion practice this is treated as a genuine hazard because of misgrouping. Beyond that, type A has subdivisions such as A1 and A2, and a person who receives a hematopoietic stem cell transplant can end up with the donor's blood type.

So even clinically, "four types" is a practical approximation. A single test taken once in a school gymnasium often gets carried through life as an identity, yet it is not rare for that verdict to be overturned by a subgroup. If the box wobbles in medicine, it is a thin place to rest an entire personality.

Where the Game Starts to Hurt

Japan has a word for it: bura-hara, blood-type harassment. There are documented cases of employers asking for blood type on application forms and in interviews, of kindergartens sorting children into classes by blood type, and of type B and AB employees being passed over for assignments. In 2004 alone, more than 70 television programs ran blood-type personality segments, prompting the Broadcasting Ethics and Program Improvement Organization to ask broadcasters to stop encouraging content that could fuel discrimination. More recently, a Japanese policy research institute has published analysis of the real-world effects of blood-type discrimination. Korea saw its own version after My Boyfriend Is Type B, with half-joking stigma attached to one group for years. The moment a line said for fun narrows someone's opportunities, it has stopped being a game.

The blood-type diet sits on the same line. Peter D'Adamo's Eat Right 4 Your Type, published in 1996, sold worldwide on the claim that each blood group needs its own menu. In January 2014 researchers at the University of Toronto analyzed 1,455 adults and found that whatever health benefits each diet produced were entirely independent of the person's blood type (PLOS ONE, 2014). The instant a seasoning for conversation is served as health or medical advice, entertainment turns into misinformation.

So How Should You Enjoy It?

Five habits keep this light and still sensible. First, aim it at yourself only. "I'm type A, so I tend to be prudent" is a question you ask yourself; "you're type A, so of course you're timid" is a verdict you hand someone else. Second, keep it out of decisions — hiring, evaluations, team assignments, turning down a date. Third, swap the label and reread. If a passage about type B reads just as convincingly with "type A" in its place, the passage was describing people, not blood groups. Fourth, take it as a question rather than a prediction: am I actually meticulous, or have I believed for years that I ought to be? Fifth, do not press others for their blood type; for someone who would rather not answer, the question itself is a weight.

People are beings far too varied to be fully held by four types, and even within the same blood type each lives in a different hue. The attitude of enjoying that variety is the way to take this game most healthily. FortuneLeaf's blood-type content, too, borrows this familiar culture to stand beside you as you joyfully call to mind the dispositions of yourself and those around you and accept one another's differences with a smile. Only, please remember that this is not science but, at most, joyful entertainment, and keep along with it the warm heart of not pinning a person down by blood type alone.